src/common/futex.cpp

   1 /*
   2  * Copyright (C) 2011 EfficiOS Inc.
   3  * Copyright (C) 2011 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
   4  *
   5  * SPDX-License-Identifier: GPL-2.0-only
   6  *
   7  */
   8
   9 #define _LGPL_SOURCE
  10 #include "futex.hpp"
  11
  12 #include <common/common.hpp>
  13
  14 #include <limits.h>
  15 #include <unistd.h>
  16 #include <urcu.h>
  17 #include <urcu/futex.h>
  18
  19 /*
  20  * This futex wait/wake scheme only works for N wakers / 1 waiters. Hence the
  21  * "nto1" added to all function signature.
  22  *
  23  * Please see wait_gp()/update_counter_and_wait() calls in urcu.c in the urcu
  24  * git tree for a detail example of this scheme being used. futex_async() is
  25  * the urcu wrapper over the futex() sycall.
  26  *
  27  * There is also a formal verification available in the git tree.
  28  *
  29  *   branch: formal-model
  30  *   commit id: 2a8044f3493046fcc8c67016902dc7beec6f026a
  31  *
  32  * Ref: git://git.lttng.org/userspace-rcu.git
  33  */
  34
  35 /*
  36  * Update futex according to active or not. This scheme is used to wake every
  37  * libust waiting on the shared memory map futex hence the INT_MAX used in the
  38  * futex() call. If active, we set the value and wake everyone else we indicate
  39  * that we are gone (cleanup() case).
  40  */
  41 void futex_wait_update(int32_t *futex, int active)
  42 {
  43         if (active) {
  44                 uatomic_set(futex, 1);
  45                 if (futex_async(futex, FUTEX_WAKE, INT_MAX, nullptr, nullptr, 0) < 0) {
  46                         PERROR("futex_async");
  47                         abort();
  48                 }
  49         } else {
  50                 uatomic_set(futex, 0);
  51         }
  52
  53         DBG("Futex wait update active %d", active);
  54 }
  55
  56 /*
  57  * Prepare futex.
  58  */
  59 void futex_nto1_prepare(int32_t *futex)
  60 {
  61         uatomic_set(futex, -1);
  62         cmm_smp_mb();
  63
  64         DBG("Futex n to 1 prepare done");
  65 }
  66
  67 /*
  68  * Wait futex.
  69  */
  70 void futex_nto1_wait(int32_t *futex)
  71 {
  72         cmm_smp_mb();
  73
  74         while (uatomic_read(futex) == -1) {
  75                 if (!futex_async(futex, FUTEX_WAIT, -1, nullptr, nullptr, 0)) {
  76                         /*
  77                          * Prior queued wakeups queued by unrelated code
  78                          * using the same address can cause futex wait to
  79                          * return 0 even through the futex value is still
  80                          * -1 (spurious wakeups). Check the value again
  81                          * in user-space to validate whether it really
  82                          * differs from -1.
  83                          */
  84                         continue;
  85                 }
  86                 switch (errno) {
  87                 case EAGAIN:
  88                         /* Value already changed. */
  89                         goto end;
  90                 case EINTR:
  91                         /* Retry if interrupted by signal. */
  92                         break; /* Get out of switch. Check again. */
  93                 default:
  94                         /* Unexpected error. */
  95                         PERROR("futex_async");
  96                         abort();
  97                 }
  98         }
  99 end:
 100         DBG("Futex n to 1 wait done");
 101 }
 102
 103 /*
 104  * Wake 1 futex.
 105  */
 106 void futex_nto1_wake(int32_t *futex)
 107 {
 108         if (caa_unlikely(uatomic_read(futex) != -1))
 109                 goto end;
 110         uatomic_set(futex, 0);
 111         if (futex_async(futex, FUTEX_WAKE, 1, nullptr, nullptr, 0) < 0) {
 112                 PERROR("futex_async");
 113                 abort();
 114         }
 115 end:
 116         DBG("Futex n to 1 wake done");
 117 }